The Electronic (Light-Sensitive) Relay

Author: E.E. Kimberly

Fig. 28-19. Inside View of Photo-Electric Relay

The light-sensitive relay has a wide field of applications, a few of which are as follows:

• Opening doors
• Counting moving parts on a conveyor belt
• Selecting objects according to color
• Counting vehicles
• Operating burglar alarms
• Leveling an elevator at any floor
• Lighting lamps on approaching darkness

These devices operate when a light beam is broken or dimmed below a minimum intensity. Fig. 28-19 is an inside view of such a device, and Fig. 28-20 is a sample of the circuit which is sometimes used.

Fig. 28-20. Electronic-Relay Circuit for Control by Light Beam

In Fig. 28-20 the contacts to be opened or closed when the phototube is darkened are at A and B. The normal at-rest condition obtains when the thyratron is conducting and so holding the relay contacts up. The thyratron conducts only when the phototube is illuminated, and so the device operates on interruption of either the light source or the power source. In operation, the phototube current, which is uni-directional, passes through resistor R₂ condenser Ci, and the potentiometer from which it receives its voltage.

Fig. 28-21. Application of Photo-Electric Tube and Light Beam for Counting Objects on a Conveyor

Condenser C₂, in parallel with R₂, is charged also ; and discharges through R₂ when the phototube half-cycle current starts to decrease. This condenser therefore helps to maintain a positive grid bias on the thyratron, and insures its conducting. The circuit consisting of C₁ R₃ and a portion of the transformer winding carries a current which produces a voltage drop across R₃. This voltage biases the thyratron grid, but is shifted in phase from the thyratron's anode-circuit voltage and is not quite sufficient to fire the thyratron. It is obvious that the anode voltage ep, even at its maximum, must not be great enough to fire the thyratron without the aid of the phototube-augmented positive grid voltage. However, when aided by the phototube-produced positive voltage across R₂, the voltage across R₃ will fire the thyratron on the positive voltage cycle. If the phototube current is interrupted because of light failure, the remaining bias is insufficient to fire the thyratron and the relay relaxes. The resistor R₁ is inserted to limit the thyratron grid current when the tube fires.

Fig. 28-22. Industrial Application of Photo-Electric Relay to Control Slack in Rolling Strip Steel

Figs. 28-21 and 28-22 show two industrial control applications of the photo-electric relay.